Fluoride glasses, which are non-oxide glasses, are usually based on heavy metal fluorides such as ZrF.sub.4, HfF.sub.4 or BaF.sub.2, but also include glasses based on AlF.sub.3 and BeF.sub.2 as well as fluoro-phosphate-based glasses. Unlike oxide glasses, fluoride glasses have a relatively high tendency toward devitrification and must be quenched rapidly from the melt to avoid crystal formation.
There are several methods which can be used to fabricate high optical quality infrared transmitting fibers based on fluoride glasses. Examples are: U.S. Pat. No. 4,343,638; U.S. Pat. No. 4,659,355; U.S. Pat. No. 4,519,826; and co-pending U.S. patent application Ser. No. 07/498,453. Glass-clad fluoride optical fibers are normally prepared by preform drawing, the fluoride glass preforms being fabricated either by rotational casting or built-in casting. Thus, molten fluoride cladding glass is cast inside a metallic mold pre-heated to the glass transition temperature (T.sub.g) and the mold is rotated and cooled so that the melt solidifies into a concentric and uniform tube or shell adjacent the internal wall of the cylindrical mold. Finally, the molten core glass which has a higher refractive index than the cladding glass is poured into the tube to form a fluoride glass preform which is subsequently drawn into fibers in a resistance or RF induction furnace.
The aforementioned Tran U.S. Pat. No. 4,519,826 discloses that the core melt may be introduced into the rotationally cast cladding using either suction or positive pressure. It is stated that the cladding shell or tube is introduced into the melt of the core glass and then sufficient pressure is applied to the core melt surrounding the inserted end to force the core melt into the cladding tube. However, this operation is carried out at the melting temperature which causes a degree of sublimation with condensates which are deposited on the inner wall of the cladding tube. These condensates represent nucleation sites which induce crystallization, especially in the case of unstable fluoride glasses.
The built-casting method is somewhat similar, except that the cladding tube is prepared by casting the cladding glass melt inside a metallic mold and subsequently upsetting the mold to allow the still molten glass to flow out of the mold, after which the core melt is then introduced into the tube as already described above. Another technique for fabricating polymer clad fluoride fiber consists of casting of the melt inside a metallic mold to form a solid fluoride glass rod, after which the rod is drawn into a fiber which is coated in line with a low refractive index polymer which acts as an optical cladding.
However, all techniques used to date to make glass clad fluoride glass preforms apply only to very stable glass compositions. For unstable compositions, undesirable crystal formation will be induced in the core during the casting of the core melt inside the cladding tube. Bubbles in the core, formed by trapped gas which originates from turbulence when the core melt is poured into the tube, represent an additional source of scattering defects.
Chalcogenide optical fibers, like fluoride glass fibers, are useful as infrared fibers such as for the power delivery of high intensity CO.sub.2 and CO lasers, but the chalcogenide glasses are also relatively unstable against crystallization. There is only one technique at present which can be used to make glass-clad chalcogenide optical fibers, such technique being based on crucible drawing of preforms as described in the paper entitled "Chalcogenide Glass Fibers for Power Delivery of CO.sub.2 Laser" by T. Nishii et al, pages 224-232, SPIE Vol. 1228 Infrared Fiber Optics II (1990).
However, there are two draw-backs to the crucible drawing approach: first, chalcogenide glasses which are based on Ge, As, Se, Sb and Te sublime at the drawing temperature (T.sub.draw) which is about 30.degree. C. to 45.degree. C. higher than the softening temperature (T.sub.s) of the glass, it being understood that sublimation is minimal at T.sub.s. In the crucible draw technique, the processing temperature has had to be kept relatively high, i.e. at T.sub.draw, thus inducing the formation of undesirable trapped bubbles at the core-clad interface due to sublimation. Furthermore, attempts to apply a vacuum in the spacing between the core rod and the cladding tube made sublimation even worse.
Second, the crucible drawing approach requires a core rod having an optically polished surface. During the polishing step, the rod surface is likely to become contaminated with oxide impurities. These impurities then become scattering defects at the core-clad interface of the preform.